This article highlights why it's hard to produce meaningful results "just flying thermal drone". Certified training in Thermography is required. "Thermography" defined by the ITC is "the process of acquisition and analysis of thermal information from non-contact imaging devices. Let’s break that down into something more practical. There are three parts here: "acquisition", "analysis" and "non-contact imaging".
Acquisition refers to the actual process of taking radiometric images. There are two key parts of knowledge required when taking thermal images: how a thermal camera works, and that long wave infrared is highly sensitive to environmental conditions. For a thermal camera to make an accurate measurement at some point int he image, at minimum area of 10x10 pixels is needed (for high resolution thermal cameras). This concept leads to a relationship between the size of the feature being imaged and the distance to the camera. If the camera is too far away, smaller details can't be accurately measured. This concept is expressed as Measurement Field of View or MFOV.
Another key thing about thermal cameras is that they don’t measure temperature. They measure differences in thermal radiation. How can temperatures be measured then? Typically, there are two kinds of infrared radiation being measured: emitted and reflected. Think of a how a concrete driveway feels hot for hours after the sun goes down. That driveway is emitting heat. That emission is the thing thermography wants to measure. Now think of the same hot driveway at night but surrounded by flood lights. The lights will produce a small amount of reflected infrared radiation which will be a part of the radiometric image. The amount of reflected radiation the thermal camera views is called total reflected apparent temperature (TREFL),and it can be measured by imaging something shiny that doesn’t emit like Reflectix. That is, when taking a radiometric image for quantitative analysis, another image must be taken of the Reflectix specifically to measure TREFL. Later when analyzing the images, TREFL is measured using the Reflectix image and the resulting value is used as a parameter when analyzing, in this case, the hot driveway.
Simply put, thermal cameras view a scene that contains emitted and reflected thermal radiation. To accurately measure emissivity, the reflected radiation must be measured and removed.
Environmental conditions play a large part in thermal image acquisition. A light wind can reduce the apparent temperature by 50%. Infrared radiation is absorbed by humidity, and enough of it can make features difficult to resolve. Finally, the time of day is important too. Solar panel inspections work best in the afternoon when the solar loading is high, and cloud cover is low. Finding water intrusions in a roof works best after sunset due to the higher thermal capacity of water than roofing materials. This was an important part of ITC certification: understanding the conditions that produce the best outcomes for radiometric imaging.
Analysis simply means understanding how to process and interpret radiometric images. Processing a radiometric image typically involves adjusting the pallet and contrast of the image to best display the subject. It also involves setting parameters for emission and total reflected apparent temperature (mentioned above). Interpretation requires a general understanding of the conditions and materials displayed in the image. There are many counterintuitive cases when the actual temperature of an object is much higher than displayed. For example, a shiny steel brace at the bottom of a hot coffee pot will show as being cold relative to the glass. This is due to the metal reflecting the surroundings more than emitting the actual temperature.
The final pillar of thermography is “non-contact imaging”. This should be intuitive. Thermal cameras remotely gather radiation and are therefore non-destructive and don’t require touching a thermometer to the subject. This most appreciated in the example of electrical component or power line inspection.
Thermography has 3 parts: acquisition, analysis, and non-contact imaging. Generally, all three parts require understanding how thermal images are affected by the camera, environment, subject emissivity and reflectivity, and finally that remote measuring keeps you safer.
